Leak detection equipment is used in a variety of industries to determine whether products are properly manufactured and assembled. Leak detection equipment is used to test individual products for the presence of leaks which would degrade the performance of the product during the products useful life. Not all leaks are, however, fatal to the performance of a product and a maximum acceptable leak is often established for a product.
The object of leak testing is to measure the rate of leakage and to determine whether the measured leak is less than the maximum acceptable leak. Any product leaking at a rate less than the maximum acceptable leak meets the performance specifications relating to the part under test.
A leak is defined as the escape or entry of a gas or liquid into a sealed enclosure. Leaks result from material defects (holes or porosity) or process deficiencies such as sealing or joining problems. The majority of leaks are not simple circular holes in thin walls which exhibit predictable performance but more often are comprised of multiple variable paths that tend to be unique to a particular test part subjected to a specific set of conditions. Leaks of these types can have low leakage rates making them difficult to measure.
The detection of a leak from a test part under pressure is difficult to measure if the anticipated rate of leakage from the part is low or the available time for the measurement is relatively short. High levels of pressurization can induce significant adiabatic heating effects which must be dissipated before an accurate measurement can be taken. The magnitude of this problem is a direct function of the mass of the pressurizing gas; thus, the volume enclosed by the test part and the pressure of the gas contained within the test part affect the magnitude and severity of the adiabatic heating effects.
Bubble testing is the most prevalent method of leak testing in industry. It consists of pressurizing the part to be tested, submersing the part in a water bath, and looking for a stream of bubbles. Although leaks as small as 0.05 standard cubic centimeters per minute (sccm) can be detected, this method has major disadvantages. It is slow, demands continuous operator attention, and usually requires drying the tested part before the part can continue in the manufacturing process. Determination of the amount of leakage flow is a difficult task.
Helium mass spectrometer leak detection is the most common method used to detect very small leaks as low as 10.sup.-11 standard cubic centimeters per second (sccs). The part under test is either pressurized internally or externally with helium or a mixture of helium and air. The helium leakage is drawn into a very low vacuum and introduced into a mass spectrometer which has been tuned to helium. The mass spectrometer output is proportional to the number of helium ions, which is a direct measure of leakage. Helium leak detection equipment is expensive and can require long test times.
Another method of leak detection is the pressure decay method. In the pressure decay method, the part to be tested is pressurized to a pressure determined by a supply pressure. Once pressurized, the part under test is sealed to maintain the pressure therein. A pressure sensor is attached to the part which measures the internal pressure of the part under test. If a leak is present, the pressure of the part will begin to decay at a rate determined by the size of the leak and the volume of the part. A test operator can determine the relative size of the leak by reading the pressure at the end of the test time and comparing it to a predetermined limit value.
The pressure sensor used in the pressure decay method is typically a gauge pressure sensor having a reference to atmospheric pressure. When the gauge pressure sensor is used at normal test pressures, the pressure change resulting from a leak test is a very small portion of the total range on the sensor, since the gauge pressure sensor measures pressure differential between the part pressure and atmosphere. Consequently, the signal from the sensor is small. In order to obtain a usable reading with this system, it is often necessary to extend the test time, particularly if large parts or small leaks are involved. In some cases, this results in unacceptably long test times.
Another method uses a mass flow leak sensor rather than a gauge pressure sensor. In precision mass flow leak testing, the mass flow leak sensor connects the test part to a non-leaking reference volume usually having substantially the same volume as the test part. Then the reference volume and the test part are pressurized to the same level. Both the reference volume and the test part are sealed off from the pressure supply. If a leak is present in the test part, the mass flow leak sensor measures flow inherent in the equalization of pressure between the test part and the reference volume. The difference in pressure causes gas to flow from the reference volume to the test item at a rate proportional to the leak.
An alternative method of leak detection measurement using a differential pressure sensor encloses the test part in a sealed bell jar. Leakage from the test part increases the internal pressure in the internal space within the bell jar and exterior to the test part. The increase in pressure relative to a reference pressure is measured by a transducer and converted to an equivalent leakage rate.
This method is less sensitive to the effects of high test pressures and is not sufficiently accurate to detect low leakage rates particularly where short test times are a requirement since the internal free volume of the bell jar can be large. Because the differential pressure rate involves the measurement of pressure at two different times and the interval between these times is a function of the transducer sensitivity, this method is often not adequate in critical applications. Leak measurements cannot be made until sufficient time has elapsed to develop a differential pressure.